1. Field of the Invention
The present invention relates to the use of voltage controlled oscillators in a polyphonic tone synthesizer musical instrument.
2. Related Applications
This invention is related to the inventors' copending U.S. patent application Ser. No. 619,615 filed on Oct. 6, 1975 entitled KEYBOARD SWITCH DETECT AND ASSIGNOR and to their copending U.S. patent application Ser. No. 603,776 filed on Aug. 11, 1975 entitled POLYPHONIC TONE SYNTHESIZER.
3. Description of the Prior Art
In electronic musical keyboard instruments of the tone synthesizer variety it has been found to be advantageous to use voltage controlled oscillators for the frequency determining elements of the tone synthesizer. These oscillators generally operate in such a manner that their generated frequency is a linear, or essentially linear, function of the control function. For generating the frequencies of the normal equal tempered musical scale, a linear relation between the voltage and frequency is not desirable; an exponential relation is required and is obtained by means such as that shown in FIG. 10.
In FIG. 10, twelve resistors R.sub.1 through R.sub.12 and switch contacts 1 through 12 are arranged such that the reference voltage is divided into twelve equal parts. Actuating a key contact on the musical instrument's keyboard produces a voltage input to Linear Adder 201 which is a linear function of the twelve notes within a musical octave. A second input, E.sub.2, is provided to Linear Adder 201 to introduce various frequency modulation effects such as vibrato and portomento. The sum of the input voltages E.sub.0 = E.sub.1 + E.sub.2 is applied to Exponential Convertor 202 to obtain the control signal voltage V = A exp(BE.sub.0). V is then used as a control signal for Voltage Controlled Oscillator 203 which generates a repetitive signal at the frequency f = KV, where K is a constant.
There are several practical limitations to the arrangement shown in FIG. 10 which require rather expensive circuit components and design complexity. The output frequency will vary with any change in the reference voltage. Therefore a precision regulated reference voltage source is required which is invariant with nominal fluctuations in power line voltages and with changes in the ambient temperature. The voltage controlled oscillator must by carefully designed with temperature compensating circuit elements to prevent temperature induced frequency drifts. A third source of frequency error is sometimes caused by the exponential convertor which must be precisely designed to provide the required exponential transfer characteristic over the operating range.
Although it is possible to design voltage controlled oscillators of the type shown in FIG. 10 which cover the five octaves of an electronic organ's keyboard range, it has been generally found that the better procedure is to design for a single octave and to switch new frequency determining component values for each of the five octaves. When octave switching of components in the oscillator is mechanized, a second keyboard contact switch is connected to each key so that octave switching information is generated.
Voltage controlled oscillators exhibit limitations when either the tone synthesizer is an integral part of a companion musical instrument such as an electronic organ or when a polyphonic tone synthesizer contains a plurality of voltage controlled oscillators. These limitations arise from the necessity of maintaining a precision of tuning pitch without having to continuously adjust tuning controls.
The present invention is particularly advantageous for use in an electronic musical instrument of the type described in the inventor's copending U.S. patent application Ser. No. 603,776 entitled POLYPHONIC TONE SYNTHESIZER.
Objects of the present invention include
i. frequency control of a plurality of voltage controlled oscillators PA1 ii. assignment in a musical tone generator of a plurality of voltage controlled oscillators less than the number of keys on the keyboard PA1 iii. automatic tuning controlled by a single reference oscillator PA1 iv. frequency ratios controlled by stored table of numbers PA1 v. economical mechanization by utilizing a plurality of voltage controlled oscillators which time share common control channels.
Other objects and features of the invention will become apparent in conjunction with the following descriptions and drawings.